Riot police, wielding batons, fired tear gas at groups of stone-throwing protesters in front of parliament while thousands of striking private and public sector workers tried to run for cover.

Union officials estimated that more than 20,000 demonstrators had taken to the streets in protest just as parliament began discussing the pension reform bill which raises the retirement age, curtails early pensions and cuts benefits.

Police estimated the demonstrators to number only 11,000.

Participation in recent protests has waned, partly as Greeks begin escaping to the islands for summer holidays.

"The only way we can get the government to listen and to change these harsh measures is if more people take to the streets," said Theodoros Pashos, a teacher taking part in the strike.

Elsewhere around the country, tourists were left stranded at Greece`s main port of Pireaus after striking workers prevented them from boarding ferries to holiday islands.

Scuffles broke out in the early hours Tuesday after 500 striking communist PAME port workers tried to prevent local travellers and tourists from entering the gates and boarding ferries.

Only seven out of the 21 scheduled ferries managed to leave Pireaus by midday and port authorities said dozens of scheduled voyages were cancelled due to the strike.

Greek workers disrupted rail and road transport and shut down public services across the country Tuesday in the 24-hour strike, the fifth major protest to take place this year since Athens unveiled austerity measures to battle its budget-deficit crisis.

Government offices, state banks as well as tax, municipality and judicial offices remained closed in protest against pension reforms, salary and pension cuts, and tax increases.

Hospitals operated with emergency staff while journalists also joined the strike, causing a virtual news blackout to go into effect across the country.

The gates of the country`s most popular tourism attraction, the Acropolis and nearby museum, also remained shut because of the walkout.

In an effort to protect the vital tourism industry, air traffic controllers opted not to join the strike at Athens` International Airport but Aegean Airlines said it cancelled 14 domestic flights and Olympic Air called off 34 flights to the Greek islands.

The changes, required by the European Union and International Monetary Fund in return for 110 billion euros ($140 billion) in emergency loans, are needed to help Greece narrow what has become the European Union`s second biggest budget deficit.

Analysts question whether Athens will be able to enforce the tough measures because it has the potential of unleashing large-scale social unrest - more so than any other austerity decision.

Its success will depend on whether the government can maintain a tough stance on implementing all areas of the reform bill, which is likely to become more difficult as unemployment rises and the country falls deeper into recession, analysts say.

The ruling Socialist government has 157 of the 300 seats in parliament, making it likely that the reforms will pass despite criticism.